When you use the GUI wizard to create a new mailbox, there is a check box that says “Specify the mailbox database rather than using a database automatically selected” .. How does the database get automatically selected? What criteria gets used?

Gather all databases in the org

Exclude any that are marked for exclusion

Exclude any that are not in the same AD site as the provisioning server

Pick one randomly – check if it’s “up” according to active manager. If yes, use it. If no, repeat step #4.

For #2 Set-MailboxDatabase -IsExcludedFromProvisioning:$True

The IsExcludedFromProvisioning parameter specifies that this database is permanently not considered by the mailbox provisioning load balancer. If the IsExcludedFromProvisioning parameter is enabled, new mailboxes aren’t added automatically to this database. You can manually add a mailbox if your role permits.